1.3.1 A Brief History on Semiconductor Photocatalysts
The word photocatalysis is a combination of two words, the prefix photo, defined as "light" and catalysis, the process of changing the rate of a chemical reaction without itself being involved in the reaction, thereby by reducing the activation energy needed for the reactant to form a product, the catalyst can increase the rate of the reaction. Semiconductor based photocatalysis under visible and ultraviolet light irradiation has been comprehensively studied for the quarter of the last century. Fujishimna and Honda in the year 1972 discovered the photocatalytic splitting of water on TiO2 electrodes. This brought about the dawn of a new era in heterogeneous photocatalysis. Photocatalytic studies have largely investigated on the purification of water by the degradation of numerous organic pollutants and toxic substances. Quantum size effects on photoreactions for semiconductor nanoparticles have only been recently studied.9
Semiconductor photocatalysts mainly comprise of metal oxides such as TiO2, ZnO, MnO2 but are not limited to such materials, Au and other noble metals have met with considerable attention as well,9 recently the use of graphene in semiconductor photcatalysts has been on the rise, with graphene providing exciting electron transport properties, which negates the drawbacks of traditional photocatalysts,10 these
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semiconductor photocatalysts can accelerate chemical reactions upon light absorption, typically sunlight. By utilizing the energy of absorbed photons, photocatalysts can be optimized to carry out a wide variety of important chemical processes such as environmental remediation – the process of purifying air or water by destroying toxic and other organic pollutants. Semiconductor photocatalysts have also found use in solar fuel cells, H2 from water or methane/methanol from CO2.12 Various hybrid assemblies have been thought of to improve the performance of semiconductor photocatalysts by using semiconductor-semiconductor, semiconductor-metal, and semiconductor-reduced graphene oxide (RGO) nano-assemblies.10 However, it is imperative for us to understand the intricate interface electron dynamics in all of these assemblies which will allow us to use these systems in various applications.
1.3.2 Photocatalytic Mechanism
The narrow band gap between the valence and conduction bands of a semiconductor have made it an ideal material for it to be used as photocatalysts. In order for photocatalysis to proceed, the semiconductors need to absorb energy equal to or more than its energy gap. This movement of electrons forms e-/h+ or negatively charged electron/positively charged hole pairs.9 The hole can oxidize donor molecules.
The excitation of electrons is accomplished by the absorption of photons of energy equal to or higher than the band-gap of their host material. This light-induced generation of electron-hole pairs is a prerequisite step in all semiconductor mediated photocatalytic processes. For example, when a photocatalyst such as titanium dioxide
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Fig. 1.2 Schematic representation of the photocatalytic mechanism
(TiO2) absorbs ultraviolet (UV) radiation from sunlight or illuminated light source (fluorescent lamps), it will produce pairs of electrons and holes. The electron of the valence band of titanium dioxide becomes excited when illuminated by light. The excess energy of this excited electron promotes the electron to the conduction band of titanium dioxide therefore creating the negative-electron (e-) and positive-hole (h+) pair as illustrated in Fig. 1.2. This stage is referred to as the semiconductor's 'photo-excitation' state. The energy difference between the valence band and the conduction band is known as the 'Band Gap'. Wavelength of the light necessary for photo-excitation is: 1240 (Planck's constant, h) / 3.2 eV (band gap energy) = 388 nm.
The positive-hole of titanium dioxide breaks apart the water molecule to form hydrogen gas and hydroxyl radical. The negative-electron reacts with oxygen molecule to form super oxide anion. This cycle continues when light is available.9
1.3.3 The Role of Semiconductor Photocatalysts in Environmental Remediation The use of a photocatalyst is not only restricted to killing bacterial cells, but
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also decomposing the cell itself. The titanium dioxide photocatalyst is very effective as an antibacterial agent, even when there are cells covering the surface and while the bacteria are actively propagating, where Se/Te-TiO2 NRs were used for photocatalysis induced actibacterial activity. The photocatalytic action also decomposes the end toxin produced at the death of cell. Titanium dioxide does not deteriorate and it shows a long-term anti-bacterial effect. Photocatalysts also act as highly efficient deodorizers.
The hydroxyl radicals produced during the photocatalytic reaction accelerate the breakdown of many Volatile Organic Compounds (VOCs) such as formaldehyde, nitrogen dioxide, urine, fecal odor, gasoline, and many other hydro carbon molecules in the atmosphere by destroying the molecular bonds. This helps combine the organic gases to form a single molecule that is not harmful to humans thus enhancing the air cleaning efficiency.9
The photocatalytic reactivity of titanium oxides finds key use for the reduction or elimination of polluted compounds in air such as NOx, and cigarette smoke. It also exhibits high photocatalytic activity in the treatment of atmospheric constituents such as chlorofluorocarbons (CFCs) and CFC substitutes, greenhouse gases, and nitrogenous and sulfurous compounds. Photocatalyst coupled with UV lights can oxidize organic pollutants into nontoxic materials, such as CO2 and water and can disinfect certain bacteria. This technology is very effective at removing further hazardous total organic compounds (TOCs) and at killing a variety of bacteria and some viruses in the secondary wastewater treatment. Pilot projects demonstrated that photocatalytic detoxification systems could effectively kill fecal coli form bacteria in secondary wastewater treatment.9